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Sample pump eliminates emissions

By Kazi Hassa, Parker Hannifin

ChemicalProcessing.com

Keywords: "process analyzer", pump and emissions

A compact system significantly improves analyzer accuracy. This innovative turnkey solution for analyzer emissions is a solid-state pump with an ultra-sensitive pressure regulator that mixes a compatible gas with the analyzer samples to achieve a constant vent header pressure — even during times of plant upset.

Effectively dealing with the effluent from process analyzers is a concern to both environmental engineers and process analyzer engineers at petrochemical plants. The effluent from process analyzers is either vented off to atmosphere or pumped to a return point such as a flare stack, process, incinerator, or analyzer catalytic burner.

The most common practice is to vent off effluent from the process analyzer to the atmosphere. While this keeps the analyzer stable and accurate because the atmosphere is generally a very stable reference point, it results in the atmospheric release of tons of process fluids/gases causing an increase in air emissions and creating an occupational safety hazard to the workforce at analyzer shelters.

Sometimes, the release of effluent from analyzer to atmosphere or process isn’t feasible because of the high sensitivity of the instruments. A good example is an oxygen analyzer. Effluents from these analyzers must be dumped into a vent header with a stable pressure. Otherwise, an unstable vent header will cause back pressure in the analyzer, resulting in measurement errors.

Catalytic burners offer one alternative solution. Their use, however, is restricted by their limited flow capacities and need for frequent maintenance. Another option — using complex vent systems — also is available, but they are highly engineered and expensive to install.

Meanwhile, air emission legislation limiting process emissions to the atmosphere has been enacted over the years, and as a result of this, through the use of the various options available to date, release of toxic substances to air from industries in North America has been reduced by 21% (see http://www.cec.org).

One innovative turnkey solution for analyzer emissions is Parker’s recently introduced Vent Master It is a solid-state pump with an ultra-sensitive pressure regulator that mixes a compatible gas with the analyzer samples to achieve a constant vent header pressure — even during times of plant upset. It provides a stable condition at the process analyzer, while reducing emission by returning effluent from the process analyzer into a plant’s flare disposal system or process. In addition, Vent Master assists in compliance with environmental legislation, and improves occupational safety for employees in and around the process analyzer shelter.

Principle of operation

The Vent Master is a completely mechanical assembly with minimal moving parts, requiring no regular maintenance. Its highly compact size ensures that it will fit easily into even the smallest of analyzer shelters. Figure 1 provides a standard view of the pump, which consists of a yellow control box as well as the header pressure regulator.

Figure 1. A typical vent control panel — out of the box.

The enclosure, measuring approximately 30 cm × 38 cm (12 in.× 15 in.), functions as a pneumatic circuit using regulators, gauges, a rotometer, and other controls. The header pressure regulator’s outlet is connected to the vent header, which is a low-pressure pipe housed within the analyzer shelter.

The Vent Master has been designed to operate with an 18 standard liters per minute (SLPM) nitrogen gas supply. However, either natural gas or other process gas, with relatively simple re-orientation of the internal components, can replace this nitrogen gas or motive force. During the basic operation, nitrogen gas enters through the Vent Master to the header pressure regulator, which maintains 1-in. water column pressure (factory set) in the analyzer shelter vent header.

The nitrogen flowing through the pressure regulator maintains constant pressure on the vent header and provides a make-up flow volume to keep a constant flow rate to the header. As a sample volume is introduced into the vent header, the pump automatically reduces the nitrogen flow to maintain a steady 18 SLPM; constant rate is preserved by an internal pneumatic circuit.

For example, if the shelter’s collective analyzer effluent entering the vent header is 14 SLP, then the make-up nitrogen flow will automatically adjust to 4 SLPM. The Vent Master is available in flow capacities of 2 SLPM, 6 SLPM, 12 SLPM and 18 SLPM. The combined vent header effluent and make-up nitrogen flow returns to the pump and the flow rate is monitored by an on-board rotometer (Figure 2).

Figure 2. The typical flow tree for a sampler: green is nitrogen, purple is the sample and red is the waste stream.

 Table 1 presents a simplified operation sequence.

Typically, vent header pressure is lower than return header pressure, which must be reversed for flow to occur from the vent header to the process return or disposal return header. An on-board eductor increases vent header pressure, inducing a vacuum, like a solid-state pump. This higher pressure at the outlet of the eductor overcomes the higher pressure in the return header; samples gases are pumped from the low pressure vent header to the higher pressure return header. In this process, the vent header maintains a constant pressure within ± 0.3 in. of water column (IWC). Constant pressure enables reliable and accurate analyzer measurements.